Modems are used for transferring information between parties on communication lines or other communication media. The modem converts information from electrical signals on the communication line to data bits, and vice versa. In the past, nearly all modems used in homes and small offices operated by dial-up over telephone lines, and were therefore constrained by telephone circuitry to operate in the “voice band,” up to about 4 kHz. Standard voice-band modems (also referred to as analog modems) are therefore limited to low data rates, typically no more than 56 kbps, as specified by the ITU-T V.34, V.90 and V.92 recommendations, for example.
Recently, new types of modems have come into use, such as Digital Subscriber Line (DSL) modems, offering much higher data rates over telephone lines. DSL operation, however, requires installation of special, costly hardware at both the customer premises and the central office of the telephone company.